@Klakla We are transitioning to a new extensions server. If you want to use Slicer-4.11 then you need to follow instructions here to manually install extensions; or upgrade to Slicer-4.13.
@Sam_Horvath When I open the extensions manager in Slicer-4.11 then a browser window pops up (I just closed it because I did not expect my browser to appear, got slightly annoyed and suspicious that my computer might have been infected by malware that wants to show content in the browser) and the “Install extensions” tab is completely empty. Is this due to having some kind of automatic redirect from the old server? Instead of this very confusing behavior, we should show a short message on the “Install extensions” tab that tells that the extensions server is moved and further instructions can be found at (link).